ModelLoader: add the possibility to compute the reduced model passing a lambda

[Nicogene]

The ModelLoader gives the possibility to compute a reduced model specifying a list of joints to consider, a file or a string.

It would be nice if it would be possible to pass a lambda that express a predicate that if for a joint it returns true it would be considered in the computation of the reduced model.

Something like this:

std::vector<std::string> considered_joints;
 for (int j_id=0; j< model.getNrOfJoints(); j_id) {
        auto joint = model.getJoint(j_id);
        joint_name = model.getJointName(j_id);
        if (joint->getNrOfDOFs() == 0) {
            continue;
        }
        considered_joints.push_back(joint_name);
}

mdlLoader.loadReducedModelFromFullModel(model, considered_joints);

Will become something:

mdlLoader.loadReducedModelFromFullModel(model, [](IJointPtr joint){ return joint->getNrOfDOFs() != 0; });

(This code is for exclude the fixed joints from the model)